Bias-resolved comb-formation regimes in a simple two-section quantum-dot mode-locked laser for isolator-free, terabit-scale O-band interconnects

Abstract

Practical O-band quantum-dot (QD) mode-locked comb sources must combine wide flat-top bandwidth, low radio-frequency (RF) timing noise, error-correctable bare-line modulation, and feedback tolerance, yet these functions are usually optimized separately and their relation to comb-formation dynamics remains unclear. Here we map the gain-current/saturable-absorber-bias plane of a simple two-section InAs/GaAs QD mode-locked laser to resolve distinct application-specific regimes. A low-injection, high-reverse-bias regime produces 0.80 ps pulses, consistent with amplitude-modulated operation, whereas a high-injection, intermediate-bias regime yields a 16.16 nm (2.75 THz) flat-top comb with 110 lines and a 14.98 ps extended waveform, consistent with a stronger frequency-modulated contribution. To our knowledge, this is the broadest reported 3 dB bandwidth among QD mode-locked comb sources. At a separate low-noise bias, the beatnote exhibits a 0.58 kHz Lorentzian linewidth and a 41.6 fs integrated timing jitter over the 4 to 80 MHz range, to our knowledge the lowest reported for a high-channel-count O-band QD passive comb. In isolator-free 25 Gb/s NRZ transmission, all 94 carriers across a 13.73 nm 3 dB band remain below the 7 percent HD-FEC threshold. Using all 110 carriers at the maximum-bandwidth bias gives a projected aggregate rate of 2.75 Tb/s. At a separate feedback-test bias, broadband mode locking is preserved and, beyond approximately -28 dB feedback, the RF beatnote enters a feedback-stabilized regime whose linewidth is reduced 42-fold. Thus, short-pulse, broadband, low-noise, transmission-ready, and feedback-stabilized operation occupy distinct selectable regions rather than a single universal optimum. This regime-resolved map links QD comb-formation physics to isolator-free, terabit-scale O-band interconnects.